The File API (application programming interface), a draft standard at the World Wide Web consortium, lets browsers handle files better. Among its abilities are uploading multiple files at once, showing thumbnail previews of images that have been selected for upload, breaking a long video upload up into chunks to protect against network interruption problems, and integrating with drag-and-drop Web applications.

While many software projects use beta testing periods to shake down their code, Mozilla isn't afraid to add new features as it goes. That can mean new ideas arrive sooner, of course, but it also can delay the completion date of the new version. What was to have been a quick Firefox 3.1 release was pushed back months as new features were added and the version ultimately was renamed Firefox 3.5.

For those who want to dig into the File interface, Mozilla offers a Web developer guide to using it.

The beta-testing periods aren't just important for debugging Firefox itself. New versions often don't work with older add-ons that people install to customize the browser, so beta testing gives some time for programmers to update those add-ons. Mike Belzner, Mozilla's director of Firefox, said 70 percent of add-ons are now compatible with Firefox 3.6.